An electroluminescence device comprising organic thin film layers including an emitting layer between an anode and a cathode which emits light from exciton energy generated by recombine of holes and electrons injected into the emitting layer has been known (see Patent Documents 1 to 8, for example).
Such an organic electroluminescence device is expected as an emitting device having high luminous efficiency, better image quality, low power consumption and excellent thin-form design, utilizing the merits as a self-emitting device. For the formation of an emitting layer, the doping method in which a host is doped with an emitting material as a dopant is known.
In the emitting layer formed using the doping method, exitons can be generated from charges injected into a host efficiently. The exiton energy of generated exitons is transferred to a dopant, whereby highly efficient emission can be obtained from the dopant.
Recently, further studies have been made on the doping method and a suitable host material has been developed in order to improve the performance of an organic electroluminescence device. As an invention which describes such a host material, Patent Documents 1 to 8 can be given, for example. Patent Documents 1 to 8 describe compounds containing a carbazole skeleton and a nitrogen-containing aromatic ring in one molecule, and compounds containing a plurality of carbazole skeletons in one molecule, as shown in the following compounds I to VIII.
The compounds I and II described in Patent Document 1 each have a structure formed by bonding of a carbazole skeleton to a benzene ring and an electron deficient nitrogen-containing hetero aromatic ring structure. A carbazole skeleton has been known as a main skeleton of a hole-transporting material as represented by polyvinyl carbazole. In contrast, an electron deficient nitrogen-containing hetero-aromatic ring structure is known as a structure having a high electron-transporting ability. Therefore, compounds I and II described in Patent Document 1 is a material produced so as to balance the charge transport by combining a hole-transporting skeleton and an electron transporting skeleton.

However, compound I has an insufficient hole-transporting ability due to the presence of only one carbazole skeleton, and hence, good emission properties cannot be obtained. In addition, although compound II has two carbazolyl groups, these carbazolyl groups are arranged with being separated into right and left with respect to the bond axis of a pyrimidine ring and a benzene ring (two conjugated aromatic rings). As a result, the overlapping of carbazole skeleton parts in a molecule is prevented, and hence sufficient hole-transporting ability cannot be obtained, whereby the position of charge recombination tends to locate towards to an anode. Therefore, good emission properties and life properties may not be obtained.
Under such circumstances, in order to develop sufficient hole-transporting ability by increasing the overlapping of molecules, incorporating a structure in which carbazole skeletons are linked into a molecule have been devised. For example, compounds III to VI described in Patent Documents 2 to 5 have the structure in which two carbazole skeletons are linked into a molecule. However, any of these compounds do not contain an electron deficient nitrogen-containing hetero aromatic ring structure. As a result, it is difficult to adjust carrier balance of holes and electrons, whereby good emission property cannot be obtained.
In addition, compound VII described in Patent Document 6 has an electron deficient nitrogen-containing hetero aromatic ring structure and a carbazole linking structure. However, the two carbazole structures are bonded through carbon on the third position and nitrogen. In this structure, two carbazole skeletons are twisted with each other, leading to deterioration of the planarity. As a result, the degree of overlapping between molecules becomes small, which leads to an insufficient hole-transporting ability. Therefore, no good emission property and life property may be obtained.
Compound VIII described in Patent Document 7 has a structure in which a bipyridyl group as a nitrogen-containing aromatic heterocyclic group is bonded to a benzene ring of a carbazole skeleton. Although this compound is used as a material for an electron-transporting layer, the performance of the phosphorescent host material is not disclosed. However, it is assumed that since the compound has a high electron-transporting ability, the carrier balance in the emitting layer is bad when used as a host material, whereby a good emission property cannot be obtained.
In Patent Document 8, a linking group is required to be present between a heterocyclic ring and a carbazole group, whereby compounds having desired properties may not be obtained (for example, having low solubility).